Colour-changing windows get a palette upgrade

A new research paper could show how to easily and inexpensively expand the colour palette for glass that changes colour based on voltage. Researchers working on nanophotonics at Rice University have released a new report detailing how the use of the hydrocarbon molecule perylene can create glass able to turn two different colours at low voltage.

“When we put charges on the molecules or remove charges from them, they go from clear to a vivid color,” said Halas, director of the Laboratory for Nanophotonics (LANP), lead scientist on the new study and the director of Rice’s Smalley-Curl Institute.

“We sandwiched these molecules between glass, and we’re able to make something that looks like a window, but the window changes to different types of colour depending on how we apply a very low voltage.”

The new colour-changing glass varies its colouration depending on polarity, meaning that a positive voltage will produce one colour and a negative voltage another. The importance of the Rice team’s new method is that until now, multicolour varieties of such glass have required a much more significant voltage, limiting their use.

The type of glass itself, changing colour as a result of applied voltage, is known as electrochromic  glass and is enjoying growing popularity. The market for such material has been estimated at a value of more than $2.5bn by 2020. Aside from the visual appeal, electrochromic glass is in demand for its light and heat-blocking properties.

Grant Stec and Adam Lauchner of Rice University’s Laboratory for Nanophotonics show off the glass. Images courtesy of Jeff Fitlow/Rice University

In 2013, then-Rice physicist Alejandro Manjavacas found that polycyclic aromatic hydrocarbons (PAHs), the family of molecules that perylene belongs to, with just a few carbon rings should produce visible plasmons. There are dozens of PAHs, all of which contain rings of carbon atoms that are decorated with hydrogen atoms.

Waves of energy that can interact with and harvest energy from passing light depending on their frequency, plasmons are a rhythmic movement in the sea of electrons that constantly flow across the surface of conductive nanoparticles. Building off Manjavacas’ work, the researchers were able to work out that the PAH plasmons were highly sensitive to charge, leading them to the conclusion they could be easily utilised for electrochromic glass.

“Dr Halas learned that one of the major hurdles in the electrochromic device industry was making a window that could be clear in one state and completely black in another,” study co-lead author Grant Stec said. “We set out to do that and found a combination of PAHs that captured no visible light at zero volts and almost all visible light at low voltage.”

The new project took almost two years to complete and relied on Stec’s design of the perylene-containing nonwater-based conductive gel that’s sandwiched between glass layers. In experimenting with the assembled window, the team found that just 4 volts was enough to turn the clear window greenish-yellow, and applying negative 3.5 volts turned it blue.

Later in the project, the team produced a second window capable of going from clear to black. In all their experiments, they found that the colour change takes a few minutes but Halas said that the transition time is improvable with additional engineering.

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